The invention describes a method for packaging polycrystalline silicon.
Polycrystalline silicon (polysilicon) is mainly deposited by means of the Siemens method from halosilanes such as trichlorosilane and subsequently comminuted into polycrystalline silicon chunks with minimal contamination.
For applications in the semiconductor and solar industries, minimally contaminated fragmented polysilicon is desired. For this reason, the material should also be packaged with low contamination before it is transported to the customer.
Conventionally, fragmented polysilicon for the electronics industry is packaged in 5 kg bags with a weight tolerance of +/−max. 50 g. For the solar industry, fragmented polysilicon in bags with a weigh-in of 10 kg and a weight tolerance of +/−max. 100 g is usual.
Tube bag machines, which are suitable in principle for the packaging of fragmented silicon, are commercially available. A corresponding packaging machine is described, for example in DE 36 40 520 A1.
Fragmented polysilicon is a sharp-edged, non-flowable bulk material with a weight of up to 2500 g for the individual Si chunks. During packaging, it is therefore necessary to take care that the material does not pierce, or in the worst case even entirely destroy, the conventional plastic bags when they are being filled.
In order to prevent this, commercially available packaging machines need to be suitably modified for the purpose of packaging polysilicon.
EP 1 334 907 B1 discloses a device for the low-cost fully automatic transportation, weighing, portioning, filling and packaging of high-purity fragmented polysilicon, comprising a feed chute for the fragmented polysilicon, a weighing device for the fragmented polysilicon, which is connected to a funnel, deflection plates made of silicon, a filling device which forms a plastic bag from a high-purity plastic sheet and comprises a deionizer which prevents static charging and therefore particle contamination of the plastic sheet, a welding device for the plastic bag filled with the fragmented polysilicon, a flowbox which is arranged above the feed chute, weighing device, filling device and welding device and prevents particle contamination of the fragmented polysilicon, and a conveyor belt with a magnetically inductive detector for the welded plastic bag filled with fragmented polysilicon, all the components which come in contact with the fragmented polysilicon being sheathed with silicon or clad with a highly wear-resistant plastic.
It has been found that with such devices, the silicon chunks often stick in the filling device. This is disadvantageous since it entails increased stoppage times of the machine.
Piercing of the plastic bag also occurs, which likewise leads to a stoppage time of the system and contamination of the silicon.
DE 10 2007 027 110 A1 discloses a method for packaging polycrystalline silicon, in which a freely suspended ready-formed bag is filled with polycrystalline silicon by means of a filling device, the filled bag subsequently being sealed, characterized in that the bag consists of high-purity plastic with a wall thickness of from 10 to 1000 μm, the filling device comprising a freely suspended energy absorber consisting of a nonmetallic low-contamination material, which is introduced into the plastic bag before filling with the polycrystalline silicon and through which the plastic bag is filled with the polycrystalline silicon, the freely suspended energy absorber is subsequently removed from the plastic bag filled with polycrystalline silicon and the plastic bag is sealed.
By such a method, which provides an energy absorber inside the plastic bag, piercing of the plastic bag can be substantially avoided. A disadvantage with this method, however, is that sticking still occurs. In this method, this primarily occurs in the energy absorber. It therefore continues to lead to production stoppages and requires mechanical interventions, which entail contamination of the silicon.
It is an object of the invention to avoid such sticking of the silicon.